I think Alex has stated in the past that Juile is easily triggered into flowering. It may be that the tree is located in a position as to get the right stimuli to flower more frequently (Proximity to cold &or dry).
It is also possible the tree is a mutant, but I wouldn't jump to that conclusion until the above circumstance is eliminated.
Additionally, a study in Australia determined that precocity of a grafted mango cultivar is dependent upon the rootstock onto which it is grafted, so the possibility also exists that the rootstock is the cause.